After a few hours of run time during setup the game lost all sync and wouldn't adjust back into lock. I brought some spare Philips KT-3 parts the following day and swapped in a replacement line frame sync daughter PCB that fixed it. I'd always had a problem with this chassis where the vertical sync would not lock until the chassis warmed up and alas the replacement sync PCB exacerbated this problem and eventually the vertical sync would not stay locked even when warm (it could be tweaked to lock, but would lose lock after a few minutes). The game was out for the remainder of the show :(
The Philips KT-3 chassis uses the same type of video input connector as the Hantarex MTC-90 (but with a different pinout) so I was able to reuse MTC-90 connectors that I'd kept from previous MTC-90 to MTC-900 conversions to make up a video input cable for my test harness.
The Philips KT-3 chassis runs off an isolated 220VAC supply. I had a spare power supply from a Zaccaria Scramble along with a power input cable made up from connectors recovered from some scrapped KT-3 chassis.
I had no spare Philips KT-3 CRT to dedicate to a test bench CRT so the plan was
to determine how close the Philips A51-572X CRT was to any of the other CRT's I already
had available. The neck pinout was compatible with the A51-211X CRT I was using for
testing MTC-90/MTC-900 FB monitor chassis according to the BK 467 CRT adaptor
guide (both using adaptor CR-25 with 6.3V heater voltage), so this was the first
one to try out.
The next step was to measure the yoke resistances of both the original A51-572X and
the A51-211X to see how close they were to each other:
|Green - Yellow||12.0 Ohms||Virtical||15.2 Ohms|
|Red - Black||2.5 Ohms||Horizontal||2.2 Ohms|
On the bench the chassis didn't exhibit any of the vertical sync lock problems seen in the cabinet. There was a wide range of vertical sync pot range that locked sync versus in the cab where it was very sensitive. I suspected that the Quasar main PCB might be the problem so I pulled that out of the cab and ran both the PCB and monitor chassis on the bench together but still saw no problems with the vertical sync. I verified the sync output from the Quasar main PCB with a scope against another Quasar main PCB and saw no differences in appearance. As far as I could tell both chassis and game PCB were working fine on the bench together :|
During the show the repeated pot tweaking had bent the adjustment pots and caused them to become lose. I found some higher quality ceramic ones with matching pinout off eBay and replaced both pots on the line frame sync board. I measured and made a note of the current pot in-circuit resistance settings and set the new pots to match after installation so that little re-adjustment would be needed.
With the chassis now confirmed working OK with the original A51-572X CRT it was time to try the chassis on the MTC-90/MTC-900 FB A51-211X test CRT. I was expecting the geometry to be mismatched due to the differences in yoke resistances and as expected it was over-scanned. Other settings for brightness, focus etc were also mismatched. I didn't attempt to correct any of these settings because they would mismatch the A51-572X CRT in the cabinet. As it was, the picture was good enough for most common fault diagnosis.
I wasn't sure what I was going to see with the game PCB and KT-3 monitor chassis installed back in the cabinet. The vertical sync problem had existed ever since I'd owned the game and had not cleared up with a cap kit or replacement sync board. On power on the monitor came up without any issues and was still working properly after an hour. It remained a mystery…